The invention relates to an internal combustion engine having two exhaust gas turbochargers, each including a turbine and a compressor, disposed in series circuits and an exhaust gas recirculation line including a non-return valve connected to an exhaust duct upstream of an exhaust gas turbine for returning exhaust gas to a compressor air intake duct.
DE 101 44 663 A1 discloses an internal combustion engine having two exhaust gas turbochargers, a first turbocharger forming a high pressure stage, and the second turbocharger forming a low pressure stage. The high pressure stage is arranged near to the engine, and the low pressure stage is in contrast located at a relatively large distance from the internal combustion engine. The compressors of the two exhaust gas turbochargers are connected in series in the intake section, and the exhaust gas turbines of the two turbochargers are likewise arranged one behind the other in the exhaust gas section. Both the compressor and the turbine of the high pressure stage are each bypassed by a bypass which can be shut off, each bypass being able to be opened or closed as a function of engine state variables and engine operating variables. When the loads and engine speeds are low, the bypass around the compressor or the exhaust gas turbine of the high pressure stage is closed and the high pressure stage is activated. As the load and the engine speed increase, the bypass around the compressor or the turbine of the high pressure stage can be opened, as a result of which the high pressure stage is deactivated and the compressor work is to be carried out solely by the low pressure stage.
Such internal combustion engines with two-stage supercharging have a transient behavior which is improved compared to single stage supercharging. However, two stage supercharging does not have any direct influence on the exhaust gas behavior of the internal combustion engine.
In order to improve the exhaust gas behavior, in particular in order to reduce the NOx emissions it is basically known to re-circulate exhaust gas from the exhaust gas section into the intake section. Such recirculation of exhaust gas is carried out in particular in the partial load operating mode of the internal combustion engine and is described, for example, in document DE 101 52 804 A1. The internal combustion engine is equipped with an exhaust gas turbocharger whose exhaust gas turbine has two different exhaust gas flows of different sizes via which exhaust gas can be fed to the turbine wheel. Each cylinder bank of the internal combustion engine is in communication with one exhaust line. A recirculation line of the exhaust gas recirculation device branches off the smaller exhaust line which has a higher exhaust gas back pressure. The recirculation line extends to an intake duct section downstream of the compressor for returning the recirculation exhaust gas to the engine. The higher exhaust gas back pressure supports the recirculation of exhaust gases into the intake section and permits exhaust gas to be re-circulated over a wide operating range of the engine.
U.S. Pat. No. 6,397,598 B1 discloses an internal combustion engine with two exhaust gas turbochargers connected in series. The internal combustion engine has two cylinder banks with three cylinders each, the exhaust gas of each cylinder bank being fed into a separate exhaust line. The first exhaust line leads into the exhaust gas turbine which is near to the engine and which has a flow connection via a further line section to the second exhaust gas turbine which is remote from the engine. The second exhaust line leads directly into the second exhaust gas turbine which is remote from the engine.
Furthermore, in U.S. Pat. No. 6,397,598 B1 an exhaust gas recirculation device is provided which branches off from the first exhaust line which is connected to the exhaust gas turbine which is near the engine. An adjustable nonreturn valve is arranged in the recirculation line of the exhaust gas recirculation device.
With this configuration it is possible to bring about different exhaust gas backpressures in the two exhaust lines, which can be utilized for improved exhaust gas recirculation. However, the adjustment possibilities are restricted to regulating the nonreturn valve in the exhaust gas recirculation line. The level of the exhaust gas back pressure, in particular in the exhaust line which is coupled to the recirculation line, cannot be influenced—with the exception of regulating the nonreturn valve in the recirculation line.
Document U.S. Pat. No. 6,378,308 B1 discloses an internal combustion engine with two cylinder banks whose exhaust gas is fed in each case into one exhaust line, each exhaust line leading into an exhaust gas flow of a turbine which is near the engine. A further exhaust gas turbine which is remote from the engine is connected downstream of the two-flow exhaust gas turbine which is near the engine. A recirculation line which leads into the intake section downstream of the compressor and which is assigned to the exhaust gas turbine which is near the engine branches off from the exhaust gas collector of one of the two cylinder banks.
Furthermore, U.S. Pat. No. 6,378,308 B1 discloses a bypass in each exhaust line for optionally bypassing the exhaust gas turbine which is near the engine. Each bypass branches off upstream of the exhaust gas turbine which is near the engine and leads again into the exhaust line downstream of this exhaust gas turbine. The bypass mass flow rate can in each case be regulated via one intermediate nonreturn valve in each bypass.
It is the object of the present invention to improve the exhaust behavior of an internal combustion engine with two-stage supercharging by simple means.